Free boundary problem
= Free boundary problem
A <free boundary problem> determines both a field and part of its domain boundary. A glacier terminus, for example, separates positive <ice> thickness from bare bed. <Mass conservation>, nonnegative thickness, a rule for <ice ablation> only where <ice> exists, and suitable interface or <entropy solution> conditions are needed; a formal zero of one <characteristic> branch alone need not give the selected physical boundary.